I can't picture how that would work. While collaborative features require that some objects are shared and synchronized, efficiency and programmer sanity rely on the fact that some objects are not. If synchronization is opt-in, how would a language integrate it any more fundamentally than a library can?

Also, CRDT's don't provide synchronization for free. They ensure that all concurrent modifications will be merged somehow. If the data being synchronized has any structure, it requires careful CRDT-aware data model design to ensure the merging is semantically reasonable (or that, in the worst case, incompatible changes produce a detectably broken state).